A NASA-supported scientist is learning how to use
carbon dioxide--the main gas in Mars' atmosphere--to harvest
rocket fuel and water from the red planet.

August
20, 2003: When astronauts first go to Mars, it'll be difficult
for them to bring everything they need to survive. Even the first
tentative explorations could last as long as two years--but spaceships
can only carry a limited amount.

"We might have to do what explorers have done for ages:
live off the land," says chemical engineer Ken Debelak of
Vanderbilt University.

Right: Exploring Mars, a painting by Paul Hudson.

Explorers on Earth could usually count on finding what they
needed. The animals might be strange, but they'd be there, and
they'd be edible. Mars is barren. But the challenge is the same.
Astronauts will want to pull what they need from the planet itself.
And although that goal seems improbable, Debelak believes it
can be achieved. He's working on a NASA project to make it happen.
The key, he says, lies in the Martian atmosphere.

It's a meager atmosphere, compared to Earth's, and it's
about 95 percent carbon dioxide (CO2). But that turns
out to be an advantage. The carbon dioxide, says Debelak, can
be used to harvest almost everything else.

Inside martian rocks and soil lies a bounty of useful elements:
magnesium and hydrogen for rocket fuel, oxygen to breathe, water
to drink. What's needed is a solvent to get them out, and that's
where the carbon dioxide comes in handy.

"When CO2 is compressed to a pressure of 73
atm and heated to 31.1 degrees Celsius, it becomes a supercritical
fluid--and a marvelous solvent," says Debelak.

A supercritical fluid is a high-pressure, high-temperature
state of matter perhaps best described as a liquid-like gas.
Almost anything can become supercritical. Water, for instance,
becomes a supercritical fluid in the high pressures and temperatures
of steam turbines. Ordinary water is a good solvent. Supercritical
water is a great solvent--maybe even a little too good. It dissolves
the tips of the turbine blades.

Left: A phase diagram. The critical point, denoted by
the big gray dot, is a special combination of temperature (Tc=31
C) and pressure (Pc=73 atm) where CO2 has
properties of both liquid and gas. Above the critical point,
it becomes a supercritical fluid. [more]

On Earth, supercritical CO2 is not used much to
dissolve things because there are less expensive, more effective
solvents close at hand. It is, however, used to remove the caffeine
from coffee beans, and sometimes to dry-clean clothes. On Mars,
Debelak believes, supercritical CO2 will play a much
more important role.

For example: Magnesium can be dissolved quite easily by supercritical
CO2, Debelak has found. "That's an experiment
that we're quite excited about at the moment," he says.
Magnesium, which is likely to be found in martian soil, ignites
easily and can be used to fuel rockets. In fact, says Debelak,
one Mars exploration scenario called for a lander to be made
of magnesium--"the legs and so on." When the astronauts
were ready to go home, "you could chop it up, pack it into
a rocket engine, and then add some other oxidizer to fire it
off." Using CO2 as a solvent, magnesium could
instead be harvested directly from Mars.

Above: A chamber containing two phases of carbon dioxide--liquid
and gas. As temperature and pressure increase (from left to right),
the two phases merge to become a supercritical fluid. [more]

Supercritical CO2 might also be used to generate
water. Certain martian rocks (like some of Earth's rocks) contain
hydrogen. When these rocks are submerged in supercritical carbon
dioxide, a chemical reaction takes place. The CO2's
carbon becomes "fixed" in the rock, leaving the oxygen
free to find another partner: hydrogen. "The process kicks
out water," marvels Debelak. "You can actually use
it to form water."

Pulling water from rocks will probably have the biggest payoff,
at least in the short term, says Debelak. In addition to drinking,
"you can split water into hydrogen for fuel, and oxygen
for breathing--or as an oxidizer for some sort of engine."
Eventually, colonists could set up plants that use CO2
from the martian atmosphere to process hundreds of kilograms
of raw material a day.

Right:
The rock-strewn terrain around NASA's Viking 2 landing site on
Mars. [more]

A supercritical fluid has some advantages over other solvents:
Its solubility changes dramatically when you alter the temperature
or the pressure. You can control it, so that sometimes it's a
solvent for a particular substance, and sometimes it's not. That
makes it easy to recover the material that has been dissolved.
Let's say you have caffeine dissolved in supercritical carbon
dioxide. To recover the caffeine (caffeine recovered from coffee
beans is often put in soft drinks), you just lower the pressure
of the CO2 and the caffeine drops out.

Currently, Debelak is trying to pin down the way a variety
of substances behave in supercritical CO2. He's looking
at which minerals are easily soluble and which are not. And if
they're not, he's trying to determine how their solubility can
be improved. Adding other substances to the CO2 sometimes
helps, he says.

Debelak's
work could be useful on Earth, too. Carbon dioxide is often spotlighted
because of its damaging role in global warming. But as a solvent,
it's benign. Many solvents common in industry are toxic. They
cause cancer, and if they get into the water system, they stay
for a long time. So there's interest, says Debelak, in learning
how to use CO2 as a 'green' alternative.